In the field of hydraulic cylinders, a variety of systems have been proposed in which the displacement of the elements of the cylinder is electrically sensed and displayed or used to perform control functions. Conventional transducers have serious deficiencies, owing in part to difficulties in mounting the units and the harsh environmental conditions to which they are exposed. Transducers used on hydraulic systems in the heavy equipment industry are particularly subject to damage from the severe work environment. Hydraulic cylinders are typically located in relatively unprotected areas and are subject to, for example, high g-forces, wide temperature variations, dust, water, debris, etc., which can result in both electrical and mechanical failure.
One attempt to provide a sensor which is particularly suitable for the heavy equipment industry uses radio frequency (RF) signals. One such system is disclosed in U.S. Pat. No. 4,737,705 issued Apr. 12, 1988 to Bitar, et al. Bitar transmits a ramping RF signal into a coaxial resonant cavity formed by the hydraulic cylinder. When the cylinder's resonant frequency is transmitted, the signal detected by a receiving antenna reaches a peak. The resonant frequency has a one to one relationship with the cylinder's extension. Thus, by determining the cylinder's resonant frequency, the cylinder's linear extension can be determined.
The peak of the received signal is detected through comparison with a threshold value. The resonant frequency is determined by modifying the measured frequency of the transmitted signal to adjust for the difference between the threshold and the actual peak. This adjustment adds error into the position determination since the difference between the transmitted signal and the actual resonant frequency varies with the resonant frequency, the accuracy of the threshold value, and the size and shape of the cylinder.
The frequency of the transmitted signal is measured by determining the period of a number of cycles of the transmitted signal. When the threshold is reached, Bitar keeps the frequency constant. This also adds error since the extension of the cylinder may be changing, thereby also changing the resonant frequency of the cavity. Additionally, error is introduced by noise in the system and frequency drift of the transmitter.
Furthermore, error is introduced through the threshold value. The received waveform will vary in magnitude and shape depending upon the resonant frequency. If a constant threshold is used, error between the transmitted signal (at which peak is detected) and the actual resonant frequency will vary at different resonant frequencies, i.e., different cylinder extensions. This varying error negatively affects the accuracy, dependability, and repeatability of the sensor.
The present invention is directed at overcoming one or more of the problems as set forth above.